NOTE ON ENDOTHELIUM. 393 



and synovial membranes is a filtration or transudation, not a 

 true secretion in the ordinary physiological sense of the word, 

 for endothelium never forms glands, whereas a share in gland- 

 formation, and, consequently, secretion, is a constant charac- 

 teristic of epithelium, whether derived from epiblast or 

 hypoblast. 



As Dr. Foster says, continuity " affords no argument 

 whatever for classing .... together. We find continuity 

 everywhere." But in the case of endothelium we find in one 

 most important instance, not only continuity, but something 

 more. What takes place in the growth and development of 

 capillary blood-vessels and lymphatics? We have in each 

 case a vacuolation and excavation of connective-tissue-cor- 

 puscles, whose protoplasm becomes ultimately changed into 

 endothelial plates, and we may find every transitional form 

 between a flattened connective-tissue-corpuscle occupying 

 the lymph canalicular system and the endothelium forming the 

 wall of the lymphatic vessel with which it communicates. 

 We have here, therefore, to do with convertibility as well as 

 continuity ; and if Dr. Foster admits that the connective 

 tissues form a natural group derived from the mesoblast, of 

 which convertibility of one member into another (well seen 

 in pathological conditions) is one of the most striking cha- 

 racteristics, I do not see how he can exclude endothelium 

 from a very close connection and relationship with the 

 members of that group. In fact, the most recent researches 

 on connective tissues have established a nearly complete 

 identity between endothelium and connective-tissue-cor- 

 puscles, and this identity is strikingly evidenced by the 

 close similarity of the pathological changes which they 

 both undergo.^ 



Now, in the case of epithelium, not only has no such 

 connection with connective tissues been observed, but 

 there are marked differences between their respective 

 pathological changes. The views of Heidenhain as to the 

 continuity of the cylindrical epithelium covering the villi 

 of the small intestine with branched connective-tissue-cor- 

 puscles in the stroma have been rendered more than 



I " Les cellules du tissu conjonctif sont plates, contieiinent des noyaux 



?lats, et sout simplement appliquees sur les faisceaux du tissu conjonctif. 

 1 serait completement impossible de reconnaitre une cellule eudotheliale 

 isolee de la plupart des cellules du tissu conjonctif." — llanvier, art. 

 " Hpithelium" 'Nouveau Dictionnaire de Medecine,' vol. xiii, p. 687. 



" Les alterations des s^reuses . . . presentent une analogic com- 

 plete avec celles du tissu conjonctif." — Cornil and Ranvier, ' Manuel 

 d'Histologie pathologique,' p. 456, 



